1. Field of the Invention
The present invention relates to zoom lens systems and image pickup apparatuses including the same suitable for video cameras, digital still cameras, broadcast cameras, silver-halide-film cameras, and the like.
2. Description of the Related Art
Recent image pickup apparatuses, such as cameras including video cameras, digital still cameras, broadcast cameras having solid-state image pickup devices and cameras used with silver-halide films, have high functionality and are of small size. Zoom lens systems serving as image taking optical systems to be included in such image pickup apparatuses are desired to be compact with a short total length and a high resolution.
Further, such zoom lens systems are desired to have a wide angle of view and a high zoom ratio.
In response to such demands, there is a known four-unit zoom lens system that includes four lens units, in which a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power are arranged in that order from an object side to an image side.
As examples of such a four-unit zoom lens system, U.S. Pat. No. 7,333,274 and Japanese Patent Laid-Open No. 8-50244 disclose rear-focusing four-unit zoom lens systems, in which a first lens unit, a second lens unit, and a third lens unit are moved to change the magnification while a fourth lens unit corrects variations in an image plane caused by the change of magnification and performs focusing.
Further, among such rear-focusing four-unit zoom lens systems, US Patent Application Publication No. 2007/0091460 discloses a zoom lens system in which a still image is obtained while the entirety of a third lens unit is shifted in a direction perpendicular to the optical axis.
In general, the size of a zoom lens system can be reduced by reducing the number of lens elements while increasing the optical power of each of the lens units included in the zoom lens system.
However, lens elements of a zoom lens system configured in such a manner tend to become thick because of the increase in the optical power of each of the lens units. Therefore, the total length of the zoom lens system may not be reduced sufficiently and correction of various aberrations may become difficult.
To realize a high zoom ratio, compactness of the entire zoom lens system, and satisfactory optical performance in the rear-focusing four-unit zoom lens systems described above, it is important to make appropriate settings for each of the lens units, including the optical power and lens configuration.
In the rear-focusing four-unit zoom lens systems described above, it is particularly important to make appropriate settings of the optical power, lens configuration, and moving distance of the first lens unit that moves during zooming, the lateral magnification of the third lens unit, and so forth. If these settings are not appropriate, it is very difficult to realize high optical performance throughout a zoom range while maintaining a high zoom ratio.